feat: add OpenSearch highlight snippets to search results

- Add highlight field to ExecutionSummary record
- Request highlight fragments from OpenSearch when full-text search is active
- Pass matchContext to command palette for display

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-core/src/main/java/com/cameleer3/server/core/search/ExecutionSummary.java

@@ -30,6 +30,7 @@ public record ExecutionSummary(
         long durationMs,
         String correlationId,
         String errorMessage,
-        String diagramContentHash
+        String diagramContentHash,
+        String highlight
 ) {
 }